IQ can be used to give scientific justification to our internalized biases.

I don't want to limit your rights, because you are X. I want to limit your rights because I belong to Y, and as Y does better than X on IQ tests, it is only prudent that we know better what is good for you. I am also not interested in listening to counter-arguments coming from people whose IQ is below 99.

Also, in extreme cases, it can be used to push further policies such as eugenics (the bad kind that everyone has in mind when they hear the word "eugenics"):

Ah... I forgot to say that X shouldn't have the right to have children. No offense meant, but we want to avoid dims out breeding brights. Also, keep your stupid daughter away from my son, as I really don't want my own children to pollute genetic purity of our kind.